The invention is directed to a method for avoiding ambiguities in a pulse Doppler apparatus, wherein:
transmission pulses are periodically transmitted with a pulse repetition time into a region having moving particles, PA1 the received echo signals from this region are resolved into their quadrature components, PA1 the autocorrelation function of one quadrature component or the echo signals and the cross-correlation function of both quadrature components are formed at the location of the shift by the pulse repetition time, and PA1 the ambiguous mean Doppler radian frequency is identified from the correlation function values at the location of the shift by the pulse repetition time. PA1 a transmitter periodically transmits transmission pulses with a pulse repetition time into a region having moving particles, PA1 a receiver with a demodulator resolves the received echo signals from this region into their quadrature components, PA1 two low-pass filters are connected to the demodulator and filter the quadrature components such that they contain only the spectrum of the pulse envelopes, so that the transmission frequency and high harmonics thereof are suppressed, PA1 two analog-to-digital converters are connected to the low-pass filters that digitize the low-pass-filtered quadrature components, PA1 two filters are connected to the analog-to-digital converters and free the digitized quadrature signals of permanent echoes, and PA1 two region or domain gates distribute signal portions of the output signals of the filters onto parallel processing channels, whereby the signal portions correspond to signal-collecting regions in the region or domain having moving particles. PA1 every processing channel comprises two correlators that calculate the autocorrelation function and the cross correlation function in an environment of the shift around the pulse repetition time, whereby the environment is limited by at least twice the pulse running time that a pulse requires in order to cover the distance that is traversed by a particle having the maximum velocity to be measured; PA1 the correlators are each respectively connected to a unit for forming the mean value that averages the correlation function of a plurality of transmission pulses; PA1 the units for forming the mean values are each respectively connected to a selection circuit, these selection circuits selecting the value at the location of the shift around the pulse repetition time from the averaged correlation functions; PA1 a shift identification circuit is connected to the units for forming the mean value, this shift identification circuit identifying the shift of the maximum of one or both correlation functions vis-a-vis the pulse repetition time; and PA1 a region-identifying circuit is connected to the shift identification circuit, this regionidentifying circuit defining a region from the shift of the maximum within which one value of the plurality of possible values of the mean Doppler radian frequency identified from the signals of the selection circuit lies, whereby the value identified in such fashion is supplied for further signal processing.
The invention is likewise directed to an apparatus wherein:
In the pulse Doppler apparatus, the Doppler frequency shift must not be greate than half the pulse repetition frequency if ambiguities are to be avoided in the measurement of center frequency or velocity. In view of the ranges of measurement practically required, this represents a problem area of the pulse Doppler technique which is to be taken seriously, particularly in cardiology. High flow rates that physiologically occur in the heart cannot be ditinguished from lower flow rates. This is all the truer the more deeply the flow lies in the body.
Given narrow Doppler spectra and only one flow direction present at one time, the range can be expanded over and above this according to U.S. Pat. No. 4,680,739, incorporated by reference herein, since the conditions for a steady course of the center frequency with time are exploited. This, however, assumes that an initial value is known.
A method of the type initially cited wherein the mean Doppler frequency and the band width are defined from the correlation functions of successive echoes is disclosed by U.S. Pat. No. 4,800,891, incorporated herein by reference. In the arc-tangent function employed therein, one quarter of the pulse repetition time is already correspondingly ambiguous beginning with .+-.90.degree.. When the information that lies in the creation of the operational sign of the quotient of numerator and denominator operational signal is utilized, then the unambiguity can be expanded up to .+-.180.degree..
Another method for identifying the mean flow rate is presented in the article by O. Bonnefous and P. Pesque bearing the title, "Time Domain Formulation of Pulse-Doppler Ultrasound and Blood Velocity Estimation by Cross Correlation" that appeared in Ultrasonic Imaging 8, pages 73 through 85 (1986). The time shift of undemodulated echo signals of successively following transmission pulses is identified therein by evaluating the correlation function, and direct conclusions about the flow rate are drawn therefrom. A Doppler spectrum, however, is not acquired. Assuming a narrow Doppler spectrum, the mean velocity component in the sound direction, and thus the center frequency of the spectrum, can in fact be identified with low precision but without a domain restriction, i.e. unambiguously. When, however, the band width of the spectrum or farther-reaching properties of the flow are to be identified, then this assumes a high precision of the time measurement and long measuring times. An analysis of the flow given realizable transmission pulse lengths and running time measurements thus encounters practical difficulties. Moreover, the formation of the correlation function requires a digitization of the undemodulated, high-frequency echo signals which, due to the high dynamic demand as a result of the simultaneously occurring permanent echoes, is difficult to realize.